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Posted by: larss Sep 29 2011, 07:48 AM

If I don't fit any gasket at the top of the cylinders (which is what every expert suggests) will this affect the type/thickness of shim used at the bottom or is the increase in compression so marginal that standard shims can be used?

This is for a all standard 1,7 D-jet engine rebuild.

/Lars

Posted by: hot_shoe914 Sep 29 2011, 07:55 AM

You have to determine what compression ratio you want then you need to measure your deck height.

Posted by: 76-914 Sep 29 2011, 07:57 AM

Lars, have you planned your engine build? i.e. compression ratio desired? If so, we need those numbers first to help. This may be all you need... http://www.aircooledtech.com/calculating_cr/
....but back to your question. Yes you will raise your compression ratio.

Posted by: Cap'n Krusty Sep 29 2011, 07:59 AM

"Every expert suggests"? What, I'm no expert, despite my 38 years of T4 engine building? I use them and I've never had a problem. If you choose not to use them, plan on calculating the CR and shimming accordingly (we do that with the gaskets, because you need to do it)> You'll need a way to accurately measure the combustion chamber volume and a way to measure the piston deck height.

The Cap'n

Posted by: larss Sep 29 2011, 08:17 AM

/Lars

Thanks guys,
I will go for standard performance and aim for getting close to the standard compression ratio (within some margin). I dont know how thick the head gasket is and if removing it will be a noticeable increase? Someone may have done this calculation before for an all standard 1,7 80Hp engine ...



Sorry Cap'n, I fully respect your opinion, havent noticed any different opinions on this until now (haven't read the treads carefully enough Im afraid).
It has been written several times that VW/Porsche issued a service message about this but can not find the original posted anywhere...

Anyhow, someone may already have done the combustion chamber measurements and calculation for a all standard 1,7 80Hp setup?


/Lars S
o

Posted by: nathansnathan Sep 29 2011, 10:50 AM

The stock 1.7 head chamber is supposed to be around 49-50 cc. The piston dome is 7cc iirc.

If all else is the same, to get the same cr as stock without running a head gasket, you'd just put that same thickness of shim under the cylinder.

Posted by: Dave_Darling Sep 29 2011, 11:07 AM

VW did issue a tech bulletin bout omitting the head gaskets on at least some of their Type IV motors. Also notching the rods to get a little more splash to the underside of the piston.

Porsche issued no such bulletin.

Many builders omit the gaskets, and use a shim of the same thickness as the gasket under the cylinder. Some builders go with the way the motors were originally built, with head gaskets.

For a quick-and-dirty build, you can use a shim of the same thickness as the gasket. To do it right, though, you need to measure all of the volumes and do a little math. Google "compression ratio calculator" for some nice tools that help the math--but there is no substitute for doing the measurements. Relatively small changes in volume can make for relatively large changes in compression ratio.

--DD

Posted by: larss Sep 29 2011, 11:31 AM

Thanks guys, this made things clear to me!

Just discovered another issue:
Valve seats for #2 cyl are not flush with the head surface (as all the other are) they are inset (dropped?) about 0,05mm.

Is this what they call "dropped valve seats" and should i dump this head?

I guess fitting new seats is no good since they will go in as far as te old ones and the problem will remain?



/Lars S

Posted by: Cap'n Krusty Sep 29 2011, 11:37 AM

You obviously don't know, because you couldn't find the technical bulletin, that the delete applies ONLY to the 2 liter busses with the modified replacement cylinder heads. It doesn't apply to the original heads, nor to the 1.7/1.8 engines, nor to the 2.0 Porsche engine. The internet is filled with people who mention the document but have failed to either read or understand it. Lotsa cases like that, and not just in the car related sites. People hear things and repeat them as if they were true. Jake, among others, leaves the gaskets out, and I'm fine with that. He has the R&D and practical experience to do that, and he has the ability to prepare ALL the necessary surfaces in order to do it right, and not to just leave them out. FWIW, if your heads are not new and untouched, the CC volume isn't going to be "standard". Surfacing the head changes the CC volume, and the amount of change depends on the depth and the number of times it's been done.

The Cap'n